OELDs have been drawing attention as promising display devices because of their capabilities of emitting light of high brightness at a low driving voltage. For the purpose of postponing the life (endurance) of OELDs, that is, suppressing increase in driving voltage and decrease in luminance with time, investigations have been conducted into various structures of hole injecting materials, hole transporting materials, luminescent materials, host materials, electron transporting materials, and electron injecting materials.
Among fluorescent devices using a single excited state for light emission so far developed for improving endurance are those containing an electron transporting compound selected from a metal complex having a nitrogen-containing ligand, a nitrogen-containing heterocyclic compound, and a silicon-containing cyclic compound and, as a host material, an anthryl compound (see, e.g., JP-A-2001-284050). A mixture of a non-polar organic compound capable of transporting both electrons and holes and, as a host, an organic compound that is more polar than the transporting compound is proposed (see, e.g., European patent application publication No. 1221473). Incorporation of a host into a luminescent device using a triplet excited state capable of high luminescence efficiency is also disclosed (see, e.g., JP-A-2002-313583, JP-A-2002-313584, JP-A-2002-313585, and JP-A-2002-319492). However, these devices still need improvements on efficiency, color purity, and endurance. Moreover, the triplet red light emitting devices using a mixed host proposed in JP-A-2002-313584 and JP-A-2002-313585 supra are unsuited to industrial production due to the complexities of the methods of production.